This invention relates to an image forming apparatus of an electrophotographic type such as a copying machine, a printer, and a FAX machine in which a charging means, an image writing means, and a developing means are disposed around an image carrying member to form a toner image, which is transferred to a transfer material and is fixed on it, and in particular, to an image forming apparatus for two-sided printing (hereinafter referred to as "two-sided image forming apparatus for simplicity's sake) capable of forming an image on each of the both sides of a transfer material.
Heretofore, in image forming for two-sided printing (hereinafter referred to as "two-sided image forming" for simplicity's sake), it has been employed a method in which an image for one side formed on an image carrying member is transferred to a transfer material and is fixed on it and the image is once received in a duplex reversible feeding apparatus, the transfer material being again fed from the duplex reversible feeding apparatus at a proper timing for another image formed on the image carrying member, and the another image is transferred onto the other surface of the transfer material and is fixed on it.
As described in the above, in this two-sided image forming apparatus, transport of a transfer material such as feeding to the duplex reversible feeding apparatus and passing twice through a fixing apparatus must be done; therefore, reliability in transport of a transfer material is low, and it has been a cause to bring about the jamming and creasing of the transfer material.
On the other hand, it has been proposed a method in which toner images are formed on the both sides of a transfer material by using an image carrying member and an intermediate transfer member and then the images are fixed simultaneously in the publications of examined patent applications S49-37538 and S54-28740, and in the publications of unexamined patent applications H1-44457, H4-214576 and H10-133430.
Further, the inventors of this application have disclosed in publications of unexamined patent applications H9-258492 and H9-258516 an image forming apparatus and an image forming method in which a plurality of means for forming a toner image composed of a charging means, an image writing means, a developing means, etc. are disposed around a photoreceptor drum (an image carrying member), and after superposed color toner images formed on the photoreceptor drum are once transferred to a belt-shaped intermediate transfer member by a primary transfer means, another superposed color toner images are formed on the photoreceptor drum, then a transfer material which is fed at a proper timing to the toner images on the photoreceptor and the toner images on the intermediate transfer member is charged by a transfer material charging means to be attracted to the intermediate transfer member, and further, after the toner images on the photoreceptor drum are transferred as an image for the front side to the front side of a transfer material which is transported by the intermediate transfer member by using a primary transfer means and the toner images on the intermediate transfer member are transferred as an image for the rear side by a secondary transfer means, the transfer material is detached from the intermediate transfer member by the curvature at the end portion of the intermediate transfer member near a fixing apparatus (a fixing means) and the charge eliminating by a transfer material detaching means to be provided at need, and the toner images on the transfer material are fixed by a fixing means to form two-sided color images.
FIG. 9(a) and FIG. 9(b) show a two-sided image forming apparatus of the embodiment 1 to be explained in detail later which is of a type in which toner images are formed on the both sides of a transfer material by using one image carrying member and an intermediate transfer member; FIG. 9(a) is a drawing showing the outline structure, and FIG. 9(b) is a drawing illustrating the transfer process in this two-sided image forming apparatus. Further, FIG. 10(a) and FIG. 10(b) show a two-sided image forming apparatus of the embodiment 2 to be explained in detail later which is of a type in which toner images are formed on the both sides of a transfer material P by using two image carrying members 110 and 210 and an intermediate transfer member 14a; FIG. 10(a) is a drawing showing the outline structure, and FIG. 10(b) is a drawing illustrating the transfer process.
In an ordinary image forming apparatus, it is a general structure that a toner image on an image carrier is transferred onto a transfer material as it is when transferring onto the transfer material, and the transfer material always lies between a transferring means and the image carrier. On the contrary, in any of the above-mentioned two-sided image forming apparatuses, toner images are formed on both sides of a sheet through the total three times of transferring, including transferring in two times at the primary transferring section and transferring in one time at the secondary transferring section. In FIG. 9, any of primary transferring A and primary transferring B represents a process to transfer a toner image on a photoreceptor drum (image carrier) by charging the reverse side of an intermediate transfer member belt, but there is a possibility that transfer efficiency varies depending on whether a sheet (transfer material) exists or not. In the primary transferring B wherein a toner image lies between an intermediate transfer member and the reverse side of a sheet, it is necessary to give consideration to effect of the foregoing. Even in the publication of unexamined patent application H10-133430 stated above, effects on the primary transferring A and the primary transferring B are indicated. However, the inventors of the invention found that there is a close connection between the primary transferring and a moving speed of the intermediate transfer member.
On the other hand, secondary transferring represents a process to transfer a toner image on the intermediate transfer member onto the reverse side of a sheet by charging the obverse side of the sheet, in which a toner image on the obverse side of the sheet is given electric charges with opposite polarity. Therefore, it was found that the toner image is feared to be disturbed, and it is necessary to consider an influence of a toner image on the opposite side of the sheet in the secondary transferring.
Further, the intermediate transfer member belt takes part in all of the three times of transferring, and it was also found that there is a close connection between the moving speed of the intermediate transfer member and control of the secondary transferring, in particular.
In both of the above-described two-sided image forming apparatus, transferring is done three times, that is, twice in the primary transfer portion and once in the secondary transfer portion to form toner images on the both sides of the paper sheet P. In FIGS. 9(a) and 9(b), both of the primary transfer A and the secondary transfer B are the processes in which toner images on the photoreceptor drum 10 (an image carrying member) are transferred by charging the rear surface of the intermediate transfer belt 14a; however, it is possible that the performance of transfer is varied by the presence or absence of the paper sheet P (a transfer material). Further, because toner images exist between the intermediate transfer member 14a and the rear side of the paper sheet in the primary transfer B, it is necessary to study the influence of this. On the other hand, the secondary transfer is a process in which toner images on the intermediate transfer member 14a are transferred to the rear side of the paper sheet by charging the front side of the paper sheet; however, because the toner images on the front side of the paper sheet are given a charge of opposite polarity, there is a possibility that the toner images are disturbed. Further, in the secondary transfer, it is necessary to study the influence of the toner images on the opposite side of the paper sheet. Furthermore, because the intermediate transfer member 14a is involved in all of these three transfer processes, it is necessary to study the influence of it.
In a two-sided image forming apparatus for forming an image on each of both sides of a transfer material by using a belt-shaped intermediate transfer member as described in the above, the toner images on the front and rear sides of the transfer material are fixed by using a fixing means composed of a pair of fixing members (a pair of fixing rollers); however, if the electric resistance of the fixing members are low, or if either or both of the fixing members are grounded, it is brought about a problem that electric charge leaks from the transfer material through the fixing means to cause the transfer of the toner images to the rear side of the transfer material not to be carried out satisfactorily.
Namely, in the conventional fixing means, what is considered is only fixing on one side of a transfer material, and even in the case of a conventional two-sided image forming apparatus, the fixing process is divided into two steps for the obverse side and the reverse side. On the other hand, in the image forming apparatus like the present invention, unfixed toner images are formed on both sides of a transfer material to be fed into a fixing means, which is quite different from the conventional image forming apparatus wherein unfixed toner images are formed only on one side of a transfer material. This causes the above-mentioned problems.
Further, it is also brought about a problem that, when the transfer material enters between the fixing members, the electric image charge of the toner particles on the transfer material is induced on the fixing members, which disturbs the toner images on the front and rear sides of the transfer material to cause a satisfactory two-sided image formation not to be performed.
Further, also in the case where the fixing members are formed of an insulating material, it is brought about a problem that electric charge is accumulated on the fixing members, and the toner images on the front and rear sides of a transfer material are disturbed by the accumulated charge to cause a satisfactory two-sided image formation not to be performed.
It is an object of this invention to propose an electrical characteristic of the intermediate transfer member required for obtaining a satisfactory images for two-sided printing (hereinafter referred to as "two-sided images" for simplicity's sake) and a transfer condition for carrying out the primary transfer and the secondary transfer in the above-mentioned two-sided image forming apparatus.
It is another object of this invention solving the above-described problems to provide an image forming apparatus capable of forming satisfactory two-sided images by preventing the leakage of electric charge from a transfer material to perform transfer of the toner images to the rear side of the transfer material satisfactorily and by preventing the disturbance of the toner images on the front and rear sides by the fixing means.
The above-mentioned objects are accomplished by any one of the following structures.
Structure (1): A two-sided image forming apparatus comprising:
(a) an image carrying member; PA1 (b) a toner image forming means for forming a toner image on the image carrying member; PA1 (c) a belt-shaped intermediate transfer body for conveying a transfer material, having a volume resistivity R (.OMEGA..multidot.cm) wherein the following expression is satisfied, EQU 10.sup.9.ltoreq.R&lt;10.sup.12 ; PA1 (d) a primary transferring means for transferring the toner image on the image carrying member onto the intermediate transfer body or a front face of the transfer material; and PA1 (e) a secondary transferring means for transferring the toner image on the intermediate transfer body onto a rear face of the transfer material, PA1 (a) an image carrying member; PA1 (b) a toner image forming means for forming a toner image on the image carrying member; PA1 (c) a belt-shaped intermediate transfer body for conveying a transfer material, having a volume resistivity R (.OMEGA..multidot.cm) wherein the following expression is satisfied, EQU 10.sup.9.ltoreq.R&lt;10.sup.12 ; PA1 (d) a primary transferring means for transferring the toner image on the image carrying member onto the intermediate transfer body or a front face of the transfer material; and PA1 (e) a secondary transferring means for transferring the toner image on the intermediate transfer body onto a rear face of the transfer material, PA1 (a) a first image carrying member; PA1 (b) a first image forming means for forming a toner image on the first image carrying means; PA1 (c) a second image carrying member; PA1 (d) a second image forming means for forming a toner image on the second image carrying means; PA1 (e) a belt-shaped intermediate transfer body for conveying a transfer material, having a volume resistivity R (.OMEGA..multidot.cm) wherein the following expression is satisfied, EQU 10.sup.9.ltoreq.R&lt;10.sup.12 ; PA1 (f) a first primary transferring means for transferring the toner image on the first image carrying member onto the intermediate transfer body; PA1 (g) a second primary transferring means for transferring the toner image on the second image carrying member onto a front face of the transfer material; and PA1 (h) a secondary transferring means for the toner image on the intermediate transfer body onto a rear face of the transfer material, PA1 (a) a first image carrying member; PA1 (b) a first toner image forming means for forming a toner image on the first image carrying member; PA1 (c) a second image carrying member; PA1 (d) a second toner image forming means for forming a toner image on the second image carrying means; PA1 (e) a belt-shaped intermediate transfer body for conveying a transfer material, having a volume resistivity R (.OMEGA..multidot.cm) wherein the following expression is satisfied, EQU 10.sup.9.ltoreq.R&lt;10.sup.12 ; PA1 (f) a first primary transferring means for transferring the toner image on the first image carrying member onto the intermediate transfer body; PA1 (g) a second primary transferring means for transferring the toner image on the second image carrying member onto a front face of the transfer material; and PA1 (h) a secondary transferring means for transferring the toner image on the intermediate transfer body onto a rear face of the transfer material, PA1 (a) an image carrying member; PA1 (b) a toner image forming means for forming a toner image on the image carrying member; PA1 (c) a belt-shaped intermediate transfer body for conveying a transfer material; PA1 (d) a primary transferring means for transferring the toner image on the image carrying member onto the intermediate transfer body or a front face of the transfer material; PA1 (e) a secondary transferring means for the toner image on the intermediate transfer body onto a rear face of the transfer material; and PA1 (f) a fixing means for interposing the transfer material to convey and for fixing the toner image transferred on both faces of the transfer material, the fixing means comprising a pair of rotatable fixing members,
wherein the following expressions are satisfied, EQU 0.2.ltoreq.I(1B)/V.ltoreq.0.7 PA2 and PA2 where I(1B) (.mu.A) represents an absolute value of a primary transfer current when the toner image on the image carrying member is transferred to the front face of the transfer material by the primary transferring means, I(2) (.mu.A) represents an absolute value of a secondary transfer current when the toner image on the intermediate transfer body is transferred to the rear face of the transfer material by the secondary transferring means, and V (mm/sec) represents a moving velocity of the intermediate transfer body. PA2 wherein the following expression is satisfied, EQU 0.12.ltoreq.I(2)/V.ltoreq.0.98 PA2 where I(2) (.mu.A) represents an absolute value of a secondary transfer current when the toner image on the intermediate transfer body is transferred to the second face of the transfer material by the secondary transferring means, and V (mm/sec) represents a moving velocity of the intermediate transfer body. PA2 wherein the following expressions are satisfied, EQU 0.2.ltoreq.I.sub.2 (1B)/V.ltoreq.0.7 PA2 and EQU 0.6.times.I.sub.2 (1B).ltoreq.I(2).ltoreq.1.4.times.I.sub.2 (1B) PA2 where I.sub.2 (1B) (.mu.A) represents an absolute value of a second primary transfer current when the toner image on the second image carrying member is transferred to the front face of the transfer material by the second primary transferring means, I(2) (.mu.A) represents an absolute value of a secondary transfer current when the toner image on the intermediate transfer body is transferred to the rear face of the transfer material by the secondary transferring means, and V (mm/sec) represents a moving velocity of the intermediate transfer body. PA2 wherein the following expression is satisfied, EQU 0.12.ltoreq.I(2)/V.ltoreq.0.98 PA2 where I(2) (.mu.A) represents an absolute value of a secondary transfer current when the toner image on the intermediate transfer body is transferred to the rear face of the transfer material by the secondary transferring means, and V (mm/sec) represents a moving velocity of the intermediate transfer body. PA2 wherein an electric voltage of the same polarity as the transfer material on the both faces of which the toner images are transferred is applied to the fixing member to be in contact with the front face of the transfer material.
0.6.times.I(1B).ltoreq.I(2).ltoreq.1.4.times.I(1B)
Structure (2): A two-sided image forming apparatus comprising:
Structure (3): A two-sided image forming apparatus comprising:
Structure (4): A two-sided image forming apparatus comprising:
Structure (5): A two-sided image forming apparatus comprising: